The application of a coating by electrodeposition involves depositing a film-forming composition to an electrically conductive substrate under the influence of an applied electrical potential. Electrodeposition has gained prominence in the coatings industry because in comparison with non-electrophoretic coating methods, electrodeposition provides higher paint utilization, outstanding corrosion resistance, and low environmental contamination. Early attempts at commercial electrodeposition processes used anionic electrodeposition, where the workpiece being coated served as the anode. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has become increasingly popular and today is the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles manufactured are given a primer coating by cationic electrodeposition.
Many cationic electrodeposition compositions used today are based on active hydrogen-containing resins derived from a polyepoxide and a capped polyisocyanate curing agent. These cationic electrodeposition compositions conventionally contain organotin catalysts such as dibutyl tin oxide and lead catalysts to activate cure of the electrodeposition composition. Because of cost and environmental considerations, the levels of these catalysts are kept low. However, low catalyst levels may lessen the cure response of a coating composition, providing weaker properties in the cured film than desired. Appearance of the cured film may also be adversely affected.
Schipfer et al. in South Africa Patent Application No. 93/2977 discloses the use of cationic electrodepositable coating compositions which contain catalysts that are salts of bismuth and carboxylic acids, in particular hydroxycarboxylic acids. The reference does not disclose the use of organosulfur bismuth compounds as catalysts in cationic electrodepositable compositions.
Karol et al. in U.S. Pat. No. 5,631,214 disclose the preparation of bismuth dialkyldithiocarbamates for use in lubricant compositions. The reference does not disclose organosulfur bismuth compounds of the type presently claimed, nor the use of such compounds as catalysts in cationic electrodepositable compositions.
Bismuth compounds such as bismuth hydroxy acids have been proposed as catalysts in aqueous electrodepositable compositions. See for example U.S. Pat. No. 5,670,441. However, such compounds are prone to hydrolysis, drastically lowering the pH of the electrodepositable composition making it more corrosive to iron pipes and pumps and yielding bismuth oxide which is insoluble and relatively ineffective as a catalyst.
Bossert et al. in U.S. Pat. No. 5,859,165 disclose the use of a metal catalyst in cationic electrocoating compositions. The metal catalyst is based on the reaction product of a metal, such as manganese, cobalt, zinc or bismuth, with a mercaptan or hydroxy mercaptan or an organic acid. Such catalysts are typically used in cationic electrocoating compositions in conjunction with organotin compounds, for example triorganotin compounds. Use of such metal catalysts is often avoided, however, due to the objectionable odor generated thereby.
It would be desirable to provide an electrodepositable composition which demonstrates acceptable cure response without loss of cured film properties or appearance and which contains catalysts that do not have the shortcomings of those of the prior art.